The present invention is directed to fuel cell combustors, and, in particular, to a system and method for injecting water or water vapor into a fuel cell tail gas combustion chamber to regulate combustion temperature.
Fuel cell systems often use a tail gas burner to combust excess stack reactants. During some operating modes, reactant mixtures exist in quantities that can lead to excessively high temperatures in the burner and other equipment downstream of a fuel cell stack. These high temperatures can lead to long term system reliability degradation and even component or system failures.
One method of countering this problem has been to construct the combustion chamber from materials that are tolerant to the high temperatures that may occur. These materials are typically either special metal alloys, ceramics or some combination thereof. However, these materials are generally expensive and/or difficult to fabricate.
Another method of countering this problem has been to cool the vulnerable walls of the combustion chamber with a heat transfer fluid. This also complicates the construction of the burner and may decrease efficiency.
A third method of countering this problem has been to flow excess oxidant through the fuel cell system. This requires a larger oxidant pumping device, which increases system cost and decreases system efficiency, since the oxidant most used in fuel cell systems, i.e., air, is not a good heat transfer medium and gasses require more energy to move than do liquids.